It is known that in the field of contactless communications, various frequencies may be used during communications between a reader and a radio tag. Thus, for example, radio tags of “RFID” (“Radio Frequency Identification”) type can communicate with an item of equipment of radio reader type according to various frequencies, for example a frequency of “HF” (for “High Frequency”) type of 13.56 MHz, or a frequency of “UHF” (for “Ultra High Frequency”) type in the 890 MHz range. The HF frequency is typically used in “NFC” (“Near Field Communication”) applications and allows exchanges of information between entities up to a distance of about ten centimeters. NFC tags are found in contactless cards, passports, etc. The UHF frequency makes it possible to read a tag several meters distant. Generally, a tag and a reader operate and exchange messages on a given frequency. There also exist radio tags and readers which operate according to various frequencies. Thus, they can communicate according to a first frequency or according to a second frequency, depending for example on the distance which separates them.
Within the framework of authentications based on asymmetric cryptography between a reader and a passive radio tag which receives its energy from the reader when the latter is situated in proximity, it is customary to implement an authentication of the tag with the reader on a first communication channel, for example a channel of UHF type. However, authentication of the reader by the tag may thereafter turn out to be impossible, the UHF communication channel possibly not being suitable for authentication of the reader by the tag. Indeed, the authentication of the reader by the tag may require the latter to undertake cryptographic computations which are expensive in terms of energy. However, the electrical power supply provided to the passive tag by the reader via the UHF channel may turn out to be insufficient when the tag needs to perform certain computations to authenticate the reader. Thus, it is customary to implement the authentication of the reader by the tag on a second communication channel, for example the HF channel. The communication on the HF channel is established when the reader and the tag are about ten centimeters apart. The communication on the HF channel allows the reader to supply the tag sufficiently with energy so that the latter may undertake the cryptographic computations allowing it to authenticate the reader. The mutual authentication between the reader and the tag is thus done in two independent phases, via two different communication channels.
During mutual authentication, when a tag is suitable for communicating on various communication channels and when various channels are actually used for this mutual authentication, it is customary to implement a first authentication session on a first communication channel and a second authentication session on a second channel. The first session corresponds for example to the authentication of the tag by the reader and the second session to the authentication of the reader by the tag. The two sessions are independent. Each of these phases makes it necessary to exchange a certain number of messages.